title: Insertion profiles of 4 headless compression screws.
abstract: OBJECTIVE
  In practice, the surgeon must rely on screw position (insertion depth) and tactile feedback from the screwdriver (insertion torque) to gauge compression. In this study, we identified the relationship between interfragmentary compression and these 2 factors.
  
  
  METHODS
  The Acutrak Standard, Acutrak Mini, Synthes 3.0, and Herbert-Whipple implants were tested using a polyurethane foam scaphoid model. A specialized testing jig simultaneously measured compression force, insertion torque, and insertion depth at half-screw-turn intervals until failure occurred.
  
  
  RESULTS
  The peak compression occurs at an insertion depth of -3.1 mm, -2.8 mm, 0.9 mm, and 1.5 mm for the Acutrak Mini, Acutrak Standard, Herbert-Whipple, and Synthes screws respectively (insertion depth is positive when the screw is proud above the bone and negative when buried). The compression and insertion torque at a depth of -2 mm were found to be 113 ± 18 N and 0.348 ± 0.052 Nm for the Acutrak Standard, 104 ± 15 N and 0.175 ± 0.008 Nm for the Acutrak Mini, 78 ± 9 N and 0.245 ± 0.006 Nm for the Herbert-Whipple, and 67 ± 2N, 0.233 ± 0.010 Nm for the Synthes headless compression screws.
  
  
  CONCLUSIONS
  All 4 screws generated a sizable amount of compression (> 60 N) over a wide range of insertion depths. The compression at the commonly recommended insertion depth of -2 mm was not significantly different between screws; thus, implant selection should not be based on compression profile alone. Conically shaped screws (Acutrak) generated their peak compression when they were fully buried in the foam whereas the shanked screws (Synthes and Herbert-Whipple) reached peak compression before they were fully inserted. Because insertion torque correlated poorly with compression, surgeons should avoid using tactile judgment of torque as a proxy for compression.
  
  
  CONCLUSIONS
  Knowledge of the insertion profile may improve our understanding of the implants, provide a better basis for comparing screws, and enable the surgeon to optimize compression.
authors.affiliation: Division of Orthopedic Surgery, McGill University Health Centre, McGill University, Montreal, Canada. adam.hart@mail.mcgill.ca
authors.forename: Adam
authors.initials: A
authors.lastname: Hart
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Harvey
authors.forename: Louis-Philippe
authors.initials: LP
authors.lastname: Lefebvre
authors.forename: Francois
authors.initials: F
authors.lastname: Barthelat
authors.forename: Reza
authors.initials: R
authors.lastname: Rabiei
authors.forename: Paul A
authors.initials: PA
authors.lastname: Martineau
mesh_terms: D001863:Bone Screws; D019245:Compressive Strength; D004867:Equipment Design; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D050723:Fractures, Bone / Q000601:surgery; D006801:Humans; D008422:Materials Testing; D021361:Scaphoid Bone / Q000293:injuries / Q000601:surgery; D019415:Torque
keywords: Headless compression screw; insertion depth; insertion torque; interfragmentary compression; scaphoid fracture